The leaching of nickel-bearing oxidic ores such as laterite ores in order to recover valuable metals like nickel and cobalt can be performed in practice in many different ways. According to certain methods, laterite ore is split into limonitic and saprolitic fractions, which are processed separately. In other methods, laterite is not separated into different fractions and a common leaching treatment is carried out instead. The amount of nickel in laterites is around 0.5-4% and that of cobalt generally less than 0.2%, but they also contain a significant amount of iron, magnesium and silicates. After the leaching stages iron is precipitated and the liquid is separated from the solids. Magnesium dissolves almost completely, thus leaving mainly iron and silicates to be precipitated.
U.S. Pat. No. 6,680,035 discloses a method in which to recover the nickel and cobalt of the laterite ore the laterite is divided first into limonitic and saprolitic components. Limonite is subjected to atmospheric leaching by means of an aqueous solution of sulphuric acid and the slurry that is formed is routed to the next stage, in which the iron in solution is precipitated with a suitable precipitating agent as jarosite. Finally, the saprolitic fraction of the ore is routed to the precipitation stage, by means of which the solution is further neutralised, but the acid concentration of the solution is regulated to be in the region of 5-30 g/l, so that the magnesium and nickel dissolve. Part of the waste residue from the precipitation stage can be recycled after solid-liquid separation back to the precipitation stage as seeds.
WO application 2006/029499 discloses a method for recovering nickel and cobalt, in which leaching takes place both atmospherically and as a pressure leach. In this method too, laterite ore is divided into limonitic and saprolitic components. Leaching of the limonitic part occurs at atmospheric pressure with a mineral acid, which is mainly sulphuric acid and partly hydrochloric acid. After the first leaching stage, the slurry is routed to pressure leaching, into which the saprolitic part of the ore is also fed. After pressure leaching, solid-liquid separation is performed in order to separate the iron-containing residue and the solution containing valuable metals from each other. In pressure leaching conditions the iron will have precipitated as hematite. In one application of the method, the iron-containing residue is recycled from the first stage of a multi-stage solid-liquid separation to the pressure leaching stage as seeds.
WO patent application 2006/000098 describes a method in which on laterite ore is subjected first to crushing, after which it is made to react with a mineral acid in a mixing drum for instance. The amount of acid is sufficient to sulphate the non-ferrous metals, but not the iron. After sulphation the hardened material is ground and leached. In one application of the method, iron-containing residue from the first of the multi-stage solid-liquid separation process is recycled to the leaching stage as seeds.
WO patent application 2008/029009 describes a method in which the limonite and saprolite components of laterite are treated together. The solids are slurried in seawater and leached by means of a solution containing sulphuric acid. Part of the slurry obtained from leaching undergoes solid-liquid separation, the underflow of which is fed back to leaching to act as jarosite seeds and the overflow is combined into the slurry exiting leaching. The slurry is neutralised to precipitate the iron, after which solid-liquid separation is performed to form an overflow solution containing valuable metals and an iron residue underflow.
In the methods described above the residue formed in solid-liquid separation is recycled to some earlier stage as seeds to accelerate the precipitation of iron. However, residue formed in solid separation contains not only iron compounds and gypsum but also the components of laterite that remain undissolved such as silicates, which hinder solid-liquid separation. For this reason, the recycling of leach residue is not the most beneficial way to control precipitation.